UserScript to swap the Facebook favicon with a custom emoji

custom_emoji.js

// ==UserScript==
// @name         Facebook Favicon Swap
// @namespace    https://tampermonkey.net/
// @version      1.0
// @description  Swaps the Facebook favicon with a custom emoji
// @match        *://www.facebook.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    // Generates a data URL for a canvas with the given emoji
    function generateEmojiFavicon(emoji, size) {
        const canvas = document.createElement('canvas');
        canvas.width = size;
        canvas.height = size;
        const context = canvas.getContext('2d');
        context.font = `${size - 4}px sans-serif`;
        context.textAlign = 'center';
        context.textBaseline = 'middle';
        context.fillText(emoji, size / 2, size / 2);
        return canvas.toDataURL('image/png');
    }

    // Create a data URL for star emoji
    const starDataUrl = generateEmojiFavicon('🔵', 30);

    // Find the existing favicon or create one if not present
    let faviconLink = document.querySelector("link[rel*='icon']");
    if (!faviconLink) {
        faviconLink = document.createElement('link');
        faviconLink.rel = 'icon';
        document.head.appendChild(faviconLink);
    }

    // Update the favicon with the star emoji data URL
    faviconLink.href = starDataUrl;
})();